The subsidies are available to low- and medium-income people who buy health insurance through state-based exchanges, which opened in October. They are administered by the Internal Revenue Service in the form of tax credits.

The exchanges have been plagued by problems with the federal website used by people to enroll in health plans. The report Tuesday by the Treasury inspector general for tax administration suggests there could be more problems with the law when people claim the credits on their 2014 tax returns.

Acting IRS Commissioner Danny Werfel said the agency has a strong, effective system for administering the credits.